Tilting elements and micromachined structures incorporating tilting elements are used in various devices and microdevices and often are essential parts of them, in particular of the tilting actuators and sensors. The tilting actuation is used, for example, for many micromirrors which operate in laser displays ([1]), optical communication applications ([2]), angular rate sensors, and radio-frequency (RF) devices. The tilting actuation is also used in magnetic disk drives. In the field of optical communication, in particular, the mirrors and large arrays of mirrors were used for all-optical cross-connects, add-drop filters, variable optical attenuators, fiber optics switches, and intersatellite communication systems. Tilting micromirrors are used as scanning micromirrors also in such fields as laser printing, pattern generation for photomask writing, barcode reading, microscopy, and in such medical applications as endoscopy and tomography.
Various actuation principles can be utilized for design of tilting actuators ([3]). These principles include thermal actuation, piezoelectric actuation, acoustic actuation, as well as somewhat more widely accepted magnetic and electrostatic actuation. The latter type of actuation is highly efficient in microscale due to favorable scaling laws. Additionally, simulation of the electrostatic actuators is relatively convenient, because of the developed modeling tools.
A large variety of design concepts for electrostatic tilting actuators has been reported. Though early tilting actuators were fabricated of polysilicon using surface micromachining ([4]), the use of single crystal silicon (Si) combined with silicon on insulator (SOI) technology has become increasingly popular due to the excellent mechanical properties of such silicon and the high robustness of such devices ([1], [2], [5]).
Tilting elements of the tilting electrostatic microactuators are typically suspended using compliant torsion axes or bending flexures; and an actuating torque is provided by an electromechanical transducer. Electrostatic actuation is widely realized not only because it generally leads to high efficiency, but also because it often can be realized by established fabrication processes and compatibly with integrated circuits.
Typically in the electrostatic actuators the necessary torque is provided either by close gap mechanism ([1]) or by vertical comb driving mechanism. In the former case the actuation is provided by an electrical force or torque acting to decrease the gap between two, a movable and a fixable, electrodes; and in the latter case the actuation is provided by an electrical force or torque acting to increase the overlap between the two electrodes. The first technique is efficient for a small range of tilting angles, and the second technique can be efficient for a somewhat larger range of tilting angles, but it usually requires rather intricate fabrication processes ([1], [5]). The close gap principle is useful also while the electrodes of the biased pair are kept parallel (in the parallel plate technique).
Examples of comb-based actuator are disclosed in [9] and [10]. More examples of comb-based actuator are disclosed in patent application publications US 2004/0184132 and US 2003/0123124.
In order to achieve large amplitudes under small excitation forces, tilting actuators are often exploited in conditions of resonant amplification. For example, this amplification is used in such devices as gyroscopes, micromirrors, electromechanical filters and mass sensors. The resonant operation of single layer SOI tilting devices was reported in [6] and [7].
To benefit from the resonant amplification, the excitation frequency should match, with high accuracy, the resonant frequency of the device. This accuracy often has to be high especially in those applications where stronger amplification is desired: when high quality factors are used the excitation frequency has to fit into a narrower resonance bandwidth. However, the resonant frequency of actuator is typically uncertain, mainly due to varying environmental conditions (e.g. temperature variations) and relatively low fabrication tolerances of micromachining. While the excitation frequency is determined by an application in which the actuator is used, it is the resonance frequency that has to be tuned to the excitation frequency rather than vice versa. Therefore, ability to adjust the resonance frequency of tilting oscillators is valuable. While in some applications the excitation frequency is predetermined, in many applications the excitation frequency changes during the operation. For this latter type of the applications the tilting resonance frequency should be adjusted dynamically.
It should be noted, however, that the achievement of static response either in single layer architecture or in a different architecture is also of interest. In particular, a tilting device with such capability can be used as a switch, manipulator, or tweezer.